Coolant Warning Light May Indicate Small Leak

Dear Car Talk

Dear Car Talk | Jan 25, 2018

Dear Car Talk:

I have a 2016 Volvo XC60. When going to start the car yesterday, a "low coolant, stop engine" warning came on the dash. I opened the hood and checked, and indeed the coolant level was about an inch below the "MIN" line. We filled the coolant, and the warning on the dashboard went away. But now I am concerned that there is a coolant leak somewhere, even though I haven't noticed any puddles under my car.

The car is under warranty, and when I called the dealer, he seemed unconcerned. He said this is something that's expected and happens in the fall because of the change in temperature, and also because the coolant sensor is really high in the reservoir, so it thinks the coolant is low when it really isn't very low. He said to just top it off and not worry about it. Is there really a phenomenon of coolant "contracting" in cold weather that would explain the low level of coolant that I clearly saw? I have never experienced this with any other car, and I am finding it hard to believe. What do you think? -- Lisa


Well, things do contract when they get cold, Lisa -- ask any guy you know who's ever gone swimming in the ocean. But I don't think the coolant would contract enough for you to lose an inch of it all at once in the overflow tank.

It's possible that you were right on the edge, and a drop in temperature put the level just below the sensor. But I think it's more likely that you are slowly leaking some coolant somewhere.

The most likely scenario is that you have one or more loose hose clamps, or something simple like that. I'd make an appointment with the dealer and tell him you want him to pressurize your cooling system, keep the car overnight and check it for leaks.

What we do is we'll park a car in an area of the garage where we know the floor is dry. Then we'll pressurize the cooling system with the cooling system tester, and run the engine until it's good and hot. Then we'll turn off the engine and let the car sit overnight with the cooling system still pressurized at about 15 psi. And if there is an external leak, we'll see it on the floor the next morning.

Then, if you run your hand along the underside of the cooling hoses, inevitably you'll feel coolant in one or two spots -- usually near some hose clamps. We'll tighten up those clamps, and that solves the problem.

Small coolant leaks like this are pretty common, Lisa. And the fact that you topped it off and it's been fine for a while suggests that if there is a leak, it's a very small and slow one, not one that's going to cause a catastrophic failure while you're waiting for your appointment at the dealership. And if we and the dealer are both wrong, hey, it's under warranty!

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